In particular, in the sector of the audio band amplifiers there are currently used, in addition to linear amplifiers, switching devices which can be schematically described as a pair of switches controlled at a high switching frequency which switch the current to the load in an alternating way. The timing of the switching instants follows a modulation criterion according to the signal to be reproduced. The output signal, consisting of the sum of the carrier signal and the modulating signal, is demodulated in a low-pass filter which extracts the low-frequency modulating signal from the high-frequency carrier. A diagram of an amplifier and of a corresponding low-pass filter of this type which are currently known is provided in FIG. 1. The numbers 1 and 3 indicate two electronic switches controlled by a PWM circuit indicated in a general way by 5. The chopped signal at the output is sent to a low-pass filter formed by a single L-C network comprising an inductance L and a capacitance C. The L-C network has a cut-off frequency of the order of 25 kHz to eliminate the high-frequency switching signal from the signal sent to the load Z, so that a signal containing only the frequencies in the audio range is supplied to the load (for example a loudspeaker).
With this system, attenuations of 12 dB/octave are obtained. To obtain greater attenuations, it is possible to dispose two or more L-C networks in cascade connection, with a consequent increase of the output impedance of the amplifier.
The system may also be of the balanced type, in other words with a bridge configuration of the type illustrated in FIG. 2, where the load Z is disposed between two pairs of controlled switches 1, 3 and 1', 3'. The control signals of the two pairs of switches 1, 3 and 1', 3' are equal but in opposite phase. The symbol L, C and L', C' indicate the components of the low-pass output filters.
By comparison with amplifiers of the linear type, switching amplifiers have the advantage of a higher theoretical efficiency, equal to 100%, since there are only two possible conditions of the switches, namely: closed switch, zero voltage, non-zero current, zero power dissipation; and open switch, non-zero voltage, zero current, zero power dissipation.
At the present time, however, this technology has considerable problems, particularly for application to the audio sector, arising from the limits of the output reconstruction filter which, in order to attenuate the switching residue, also has drastic effects in the pass band, with natural resonances, distortion, and an increase in output impedance at high frequencies beyond the acceptable limits. The limits imposed by the output reconstruction filter are principally as follows:
a) increase of the output impedance of the filter as a function of the number of networks and the decrease of the cut-off frequency of each network; PA1 b) high variation of the frequency and phase response characteristics as a function of the load impedance; PA1 c) insufficient attenuation of the switching residue in cases in which the ratio between the pass band and the switching frequency is high. This problem arises whenever a compromise between quality and switching losses is sought; PA1 d) increase of the phase rotation of the output signal in proportion of the number of L-C networks used; PA1 e) increase of the resistive part of the filter with an increase in the number of L-C networks which leads to an increase in the conduction losses in the filter and consequently a loss of efficiency; PA1 f) increase of the volume and total cost of the filter with an increase in the number of L-C networks for a given switching frequency; PA1 g) resonances in the pass band which are dangerous for the operation of the amplifier in no-load conditions.